A green approach for assembling graphene films on different carbon-based substrates and their electrocatalysis toward nitrite

Abstract

Here, we demonstrate the fabrication of reduced graphene oxide films on different carbon-based substrates including glassy carbon electrodes (GCEs), graphite electrodes, and carbon paste electrodes through a green approach via a direct electro-deposition technique. The resulting electrochemically reduced graphene oxide (ERGO) films have been investigated by scanning electron microscopy, cyclic voltammetry and electrochemical impedance spectroscopy. The as-prepared ERGO film modified electrodes show different and significant electrocatalytic activity toward nitrite oxidation. Among them, the ERGO film modified GCE (ERGO/GCE) has been proven to function as electron transfer mediator and possess high electrocatalytic activity, stability and sensitivity, which might be attributed to the unique structural features of ERGO/GCE.

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Nowadays, improving the sensitivity, selectivity and stability of electrochemical sensors have been the focus of considerable research.^1–7 With the increasing demand for ultralow amount analyte detection, many attempts have been focused on the substrate modification with carbon nanomaterials such as carbon nanotubes (CNTs),^7,8 ordered mesoporous carbons (OMCs),^9,10 graphene (GN),^5,11–14 and its composites.^15–18 As a unique and tempting sensing material for electrocatalysis and biosensors, GN has attracted tremendous attention from both the experimental and theoretical scientific communities. This is mainly due to its unique nanostructure with lots of excellent properties, especially the fact that each of its atoms is a surface atom and the charge transport through a graphene sheet is highly sensitive to target molecules.^{6,13,17,19,20}

For electrochemical sensing applications, an effective strategy for depositing active materials onto a substrate is most crucial.^11,21,22 There are mainly two ways to gain GN modified electrodes, directly dropping methods and indirectly electrochemical reduction methods. As far as the former, GN were directly dropping and casting on a electrode surface, usually suffering the need to obtain pristine GN via various methods such as chemical reduction,^17,23,24 where might be suffered to the new situation that excessive reducing agents or some toxic solvents are used and therefore result contamination of the product.^25–27 Otherwise, for the indirect electrochemical reduction method, two typical steps were often adopted to gain expected GN-based electrochemical sensors. Initially, graphene oxide (GO) precursor is modified on the substrate surface by mechanical dropping or casting technology. Consequently, it would be subjected to electrochemical reduction at a given potential.

Recently, many efforts have shown that electrochemical method is an effective tool to modify electronic states via adjusting the external power source to change the Fermi energy level of electrode materials surface.^28–32 Therefore, it has been widely used in the past years. More and more electrochemical sensors have been explored, where electrochemically reduced graphene oxide (ERGO) with good response and selectivity were employed for modifying substrate. Further insights confirmed that electrochemical method is a well and, being simple, rapid, and efficiently reduces the oxygen-containing functional groups because of highly negative potential were employed.^28,33 But it very often involves sophisticated steps, and has poor stability for the step of mechanical dropping. Thereby might limit their application in the electrochemical sensors.^34,35

In order to improve the stability and sensitivity of electrochemical sensors based on ERGO films, many efforts have been made during the past years.^{1,2,5,6,33,36,37} However, it can be found that the indirectly electrochemical reduction strategy for ERGO-based electrochemical sensors preparation lack control over the film thickness and, more notable, the resulting ERGO films are not easy to separated with others coexisting components since they are mostly located on the surface of ERGO films.³⁵ Therefore developing a simple method for preparing controlled ERGO film modified electrode is a priority, which is critical for the progress in this research area to continue.

Recently, Xia and Luo et al. reported that ERGO films can be prepared on electrodes surface directly from GO dispersions by one-step electrodeposition technique.^28,34 By which, the traditional indirect electrochemical reduction method of controlled ERGO film preparation was further simplified and, importantly substituted by a simple method. Shortly thereafter, based on this finding, Luo et al. further developed a simple strategy for the synthesis of ERGO/Au nanocomposite film via using a co-electrodeposition technique.³⁵ After that, various electrochemical methods have been developed for preparing ERGO modified electrodes via one-step electrodeposition technique. For example, a typical pulsed potentiostatic method was employed by Ye et al. to gain ERGO films modified glassy carbon electrode (GCE) from GO dispersions and the resulting materials showed enhanced electron transfer properties for the target of sophoridine.³⁸ Therefore, one-step electrodeposition technique has been proven to be very effective and useful for the ERGO-based electrochemical sensors preparation. It has several clear advantages: no toxic solvents are used and therefore will not result in contamination of the modified surface; ERGO–metal-composite can be achieved easily by a simple electrochemical scan. As a green, simple, fast tool for the preparation of various ERGO and/or ERGO-based composites, would enlarge the application range of ERGO-based sensors in electroanalytical chemistry.

Here, to confirm the reliability and popularity of as-proposed method, three kinds of ERGO films modified surface on different carbon-based substrate including GCE, graphite electrode (GE), and carbon paste electrode (CPE) were performed. The approach not only show that ERGO films can be gained on different carbon-based electrodes directly from GO dispersions by one-step electrodeposition method, but also explore that there is a certain difference on the ERGO films of the different carbon-based substrates and, the resulted materials show a significant difference on the stability and sensitivity for the electrocatalytic oxidation of nitrite.

GO was purchased from Nanjing XFNANO Materials Tech Co., Ltd., (Nanjing, China). All other chemicals were of analytic grade and used as received. Deionized water was used throughout and all the experiments were performed at room temperature. Electrochemical measurements such as cyclic voltammograms (CVs), differential pulse voltammetrys (DPVs) and chronoamperometry were performed on a CHI660 electrochemical station (CHI Instruments Inc., USA) with a three-electrode system where bare GCE, CPE, GE, and the corresponding ERGO films modified electrodes were used as working electrode, Pt wire and Ag/AgCl (saturated KCl) as auxiliary and reference electrodes, respectively. Electrochemical impedance spectroscopy (EIS) measurements were performed on a VMP2 Multi-potentiostat (Princeton Applied Research, USA) using an AC signal of 5 mV amplitude at the formal potential of the redox couple over a wide frequency range. The surface morphology of the as-prepared electrodes was observed using an Ultra Plus scanning electron microscopy (SEM, Zeiss, Oberkochen, Germany) equipped with an energy-dispersive X-ray spectrometer (EDX) (Aztec-X-80, Oxford), operating at accelerating voltage of 5 kV. Raman spectra of the samples were attained with a Invia Raman Microscope (Renishaw) with a HeNe Laser excitation at 633 nm with a power of 5.0 mW. X-Ray photoelectron spectroscopy (XPS) were collected using a PHI5702 (USA), equipped with an aluminum/magnesium dual anode and a monochromated aluminum X-ray sources. The samples employed for Raman and XPS characterization were gained by collecting the dispersed sample via sonication of the film electrodes.

For the preparation of GO suspension solution (1 mg mL⁻¹), GO as purchased was dispersed in deionized water by ultrasonication for 1 h to get a homogenous dispersions. Prior to one-step electrodeposition, the GCE, GE, CPE was in turn polished with 0.3 and 0.05 μM alumina powders, respectively, followed by sonication with acetone, ethanol, and water, respectively. Afterwards, the ERGO films modified carbon-based electrodes (denoted as ERGO/GCE, ERGO/GE, ERGO/CPE) were obtained after the corresponding bare electrode was immersed in the GO homogenous dispersions and suffered cyclic voltammetric reduction with a magnetic stirring and N₂ bubbling.

In this work, CVs were employed to produce stable ERGO films onto different carbon-based electrodes surface. The electrochemical setup and CVs of different carbon-based electrodes (CPE, GE, and GCE) in the GO homogenous dispersions are illustrated in Fig. 1. The experimental setup of electrochemical reduction of GO on the different bare carbon-based electrodes is illustrated in Fig. 1a. The corresponding CVs were recorded in nitrogen-saturated 0.1 M KCl electrolyte at a scan rate of 25 mV s⁻¹ with a potential range from 0.6 to −1.4 V. As shown in Fig. 1b–d, where the typical CVs of GO electrolysis can be clearly observed including one anodic peaks (I) and two cathodic peaks (II and III) during the process. The cathodic peak (III) with a large reduction current should be due to the reduction of the irreversible surface oxygen groups since the reduction of water to hydrogen occurs at more negative potentials (e.g., −1.5 V).²⁸ Clearly, the reduction currents of irreversible cathodic peak (III) regularly increased with successive sweeps, highlighting the persistent deposition of ERGO films onto electrode surface, for the reason that ERGO films with higher conductivity were achieved by employing CVs.³⁵ Moreover, the redox waves (peaks I and II) corresponding to the phenolic hydroxyl groups on graphene planes that are too stable to be reduced by the cyclic voltammetry method.³⁴ It is worth mentioning here that the redox waves shown in Fig. 1b–d remind us of the partly reduction of GO by CVs, which are ascribed to hydroxyl groups, residual carboxylic acid and epoxy groups on the ERGO films.^35,39,40 It is clearly that the potential position of peak II for the GE is more positive than the other two electrodes, indicating a weak electrochemical reduction ability toward GO, might be due to the unique properties of GE and its active area, which will be discussed in the following.

Fig. 1 (a) Experimental setup schematic of electrochemical reduction of GO. Cyclic voltammograms of 1 mg mL⁻¹ GO in 0.1 M KCl at bare CPE (b), GE (c), GCE (d), respectively. Scan rate: 25 mV s⁻¹.

As well known, many factors can influence the electrochemical reduction process of GO. For example, GO concentration, electrochemical deposition potential range and scan cycles, reduction temperature, pH values of supporting electrolyte solutions, stirring speed of solution and so on.^28,34,35 To the best of our knowledge, the factors of substrate electrode on the GO electrolysis were not involved and reported until now. Here, the effect of various carbon-based substrates on the ERGO films preparation was proposed and confirmed for the first time. For the CPE (Fig. 1b), the CVs show a large cathodic current at −1.0 V with a starting potential of −0.70 V. In the second and successive cycles, the reduction current at −1.0 V increases considerably, demonstrating that the deposition of conducting ERGO on the CPE has indeed been achieved.³⁴ Besides, the redox waves (peak I and II) corresponding partly reduction of GO can be observed, where peak II was seen with a weak and lower current. Compared to CPE, the CVs of GE and GCE in the same condition, not only show the typical CVs of one anodic peak (I) and two cathodic peaks (II and III), but also demonstrate the different capability for GO electrodeposition. As shown in Fig. 1c, in contrast to that on CPE, a large reductive current at more positive potential of −0.90 V with a starting potential of −0.60 V is observed during the GE process. In addition, well redox waves of peak I and II were clearly seen, suggesting a typical result of partly reduction of GO by CVs. As such, shown in Fig. 1d, for the GCE process, a large cathodic current at about −0.90 V with a starting potential of −0.55 V is shown. Obviously, the starting potential of large reductive current is observed at more positive potential for the GCE than others, and possesses a weakly reductional peak (II) with rather negative potential, demonstrating that the formation of ERGO films on GCE was much easier than that of CPE and GE, which could be attributed to the structural difference of different substrates. Meanwhile, it can be found that the persistent increase of the cathodic peak currents with successive potential scans have attained for all of carbon substrates employed here, suggesting that the deposition of ERGO films on bare CPE, GE, and GCE from GO homogenous suspension has indeed been achieved via one-step electrodeposition method using CV. Furthermore, it also indicates that there are some differences for the capability of three carbon-based substrate electrodes toward the direct electrochemical reduction of GO from its homogenous dispersions by the proposed electrochemical method.

Fig. 2 shows typical SEM images of before and after ERGO films modified carbon-based electrodes. The images of bare CPE, GE, and GCE are illustrated in Fig. 2a–c, respectively. As can be seen, the bare CPE, GE, and GCE present a well-defined and smooth surface nature. Certainly, the apparent difference on the surface roughness is still existed. After modified with ERGO films, the situation of the substrate surface was underwent great changes. As depicted in Fig. 2d–f for the ERGO/CPE, ERGO/GE, and ERGO/GCE, respectively. Obviously, the general morphology of ERGO films with clearly crumpled, wrinkled and flake-like structure on different substrate surface were presented. We can conclude that electrodeposition of ERGO films from GO homogenous dispersions can occur via as-proposed one-step electrodeposition on three of carbon-based surfaces. In addition, SEM micrographs demonstrated that, at the same condition such as GO concentration, electrochemical deposition potential range and scan cycles, reduction temperature, pH values of supporting electrolyte solutions, stirring speed of solution, the as-prepared ERGO films on different carbon-based have been indeed achieved.

Fig. 2 SEM images of bare CPE (a), GE (b), GCE (c), ERGO/CPE (d), ERGO/GE (e), ERGO/GCE (f).

To further confirm it, the EDX results accompanied with SEM are provided in Fig. S1.† As shown, for GO modified surface (GCE employed as a substrate example), the C-ratio-O content is about 0.986. And it was determined to be about 9.74, 7.87 and 51.63, for the as-prepared ERGO films on CPE, GE, and GCE, respectively. Though showing a partly reductive behaviour toward GO for three of carbon-based substrates, there is also an apparent difference. From the images, it is clearly that GO can be effectively electrochemical reduced via as-proposed method. As such, the Raman and XPS experiments were performed and further proved it. As seen in Fig. S2 and S3,† here the GCE employed as an example of carbon-based substrate, the as-prepared ERGO films (under 10 CVs cycles) were characterized to confirm the feasibility of as-proposed method. It can be seen from Fig. S2† that the associated broadening of two familiar bands (D and G mode peak) were clearly observed in Raman spectroscopy of GO and ERGO. Furthermore, the I_D/I_G of the GO and ERGO is ca. 1.30 and 0.88, respectively, indicating that as partly reductive method, the as-proposed method with controllable preparation (proved by electrochemical method and see in Fig. S4†) and without usage of toxic solvents, was proved to be an effectively strategy to gain ERGO modified electrodes. XPS was employed to further probe and quantify the subsequent GO reduction. As shown in Fig. S3,† it is clear that the C/O atomic ration calculated from the XPS results is increased from C/O = 0.94 for GO up to C/O = 5.50 for ERGO, demonstrating the degree of oxidative functionality is obviously reduced in the as-prepared ERGO than in the GO.⁴¹

To confirm the electrochemical characteristics of as-prepared ERGO obtained on different substrates, the electrochemical behaviors of ERGO films modified carbon-based electrodes were performed at controlled potentials by CVs and DPVs using K₃[Fe(CN)₆]/K₄[Fe(CN)₆] as an electrochemical probe. Initially, the electroactive surface area of three bare and ERGO films modified carbon-based electrodes were measured in 5 mM K₃[Fe(CN)₆] solution containing 0.1 M KCl according to the Randles–Sevcik equation (formula (1)) by reported method.^42,43

where A is the electroactive surface area (cm²), D is the diffusion coefficient of the probe molecule in solution (cm² s⁻¹), n is the number of electrons participating in the probe reaction, V is the scan rate of the potential perturbation (V s⁻¹), and C is the bulk concentration of the probe (mol cm⁻³). Here, the correspond parameters of D and n is (6.70 ± 0.02) × 10⁻⁶ cm² s⁻¹ and 1 for the employed probe of potassium ferricyanide, respectively. Therefore, by plotting the values of I_p vs. v^1/2, the linear relationship was observed and the electroactive surface area (A) was calculated from the slope (see Fig. S5–7 in ESI†). The electroactive surface area of electrodes were gained in the trend of bare GE (0.066 cm²) < bare CPE (0.074 cm²) < bare GCE (0.088 cm²) < ERGO/GE (0.192 cm²) < ERGO/GCE (0.202 cm²) < ERGO/CPE (0.490 cm²), suggesting that ERGO films uniformly distributed on the carbon-based electrode surface and could enlarge the electroactive surface area. It can be found that the electroactive surface area of ERGO films modified CPE, GE, and GCE is about 6.62, 2.90, and 2.30 times than that of the corresponding bare electrode, respectively, resulted showing different electrocatalytic ability toward the probe and target.

CVs and DPVs were measured to examine the electrocatalytic ability of various ERGO films toward the probe, shown in Fig. 3. As can be seen, after the ERGO coating achieved, the redox peaks currents of probe were all increased, confirming that ERGO films were successfully modified onto the substrate surface and thereby accelerating the electron transfer (ET) between the probe and modified surface. The values of peak potentials and currents of probe (E_p, I_p) for before and after ERGO films modified surface are listed in Table S1,† respectively. As seen, for the CV responses, the peak-to-peak separation (ΔE_p) of probe is 180, 105 and 95 mV at the bare CPE, GE, and GCE, respectively. However, it decreased to 136, 103 and 85 mV for the ERGO films modified CPE, GE, and GCE, respectively. In addition, the response currents of the probe presented by CVs also increased than bare electrode. Especially, as seen from Table S2,† it can be observed that the DPV responses currents of probe on three of ERGO films modified surface were much higher than that on the bare electrode, where the modified CPE, GE, and GCE is about 3.57, 1.63, and 2.16 times of that on the corresponding bare electrode, respectively. It indicates that the ET of probe and modified electrode surface was significantly accelerated after ERGO films are modified and existed on the substrate surface.

Fig. 3 Cyclic voltammograms at the CPE (a), GE (b), GCE (c) and differential pulse voltammetrys at the CPE (d), GE (e), GCE (f) of 5 mM K₃[Fe(CN)₆]/K₄[Fe(CN)₆] in 0.1 M KCl, respectively (red curves for the bare electrode, black curves for the ERGO modified electrode).

The electrical conductivity is perhaps the best indicator of the extent to which GO has been reduced to ERGO. Here, EIS was used as a powerful analytical tool for probing the interfacial properties of as-prepared materials. The electrical conductivity of as-prepared modified electrodes were investigated by EIS using 5 mM K₃[Fe(CN)₆]/K₄[Fe(CN)₆] (1 : 1) in 0.1 M KCl solution and depicted in Fig. 4. The impendence data were obtained by fitting to the R(Q(RW)) equivalent circuit model (inset of Fig. 4) using the ZsimpWin program, where R is the resistance to charge transfer, Q is the constant phase angle element, and W is the Warberg-type impedance. The diameter of semicircle corresponds to the interfacial electron transfer resistance (R_ct). As shown in Fig. 4, the typical impedance with a semicircle can be clearly seen for the bare and ERGO modified CPE, GE, and GCE, respectively. The value of interfacial ET resistance (R_ct) can be estimated by ZsimpWin to be 233.6, 327 and 260 Ω for the bare CPE, GE and GCE, respectively. After the ERGO films modified, the value of R_ct of the corresponding modified electrode decreased to 5.927, 10, and 11.4 Ω, respectively, revealing the low ET resistance of the ERGO films modified surface, which are agreement with previous literatures.^26,44–46 All the results not only demonstrate that ERGO films were indeed modified on the carbon substrate surfaces via the proposed method, but also explore different electrical conductivity toward the probe molecules for the as-received ERGO films. Since the ET resistance (R_ct) of the bare and ERGO films modified electrodes was gained, the apparent ET rate constant (k_app) of the probe molecule on different electrodes was also calculated according to the literature using the equation by Lakshminarayanan (formula (2)).⁴⁷

where k_app is the apparent electron transfer rate constant, R is the gas constant, T is the absolute temperature, F is Faraday's constant, R_ct is the interfacial ET resistance, and C is the concentration of the redox probe of K₃[Fe(CN)₆]/K₄[Fe(CN)₆]. The values of k_app and related parameters of the different electrodes are tabulated in Table S3.† Compared to the bare electrodes, the ERGO films can accelerate the apparent ET rate of the probe due to its large number of edge-plane defect sites.^{38,43,48–50} Furthermore, the fastest ET rate of the probe (k_app, 8.80 × 10⁻⁶) was calculated for the ERGO/CPE, which can be attributed to its low interfacial ET resistance and high electroactive surface area (R_ct, 5.927 Ω and A, 0.4900 cm²), indicating ERGO/CPE produces an obvious improvement in interfacial ET. All the results are in agreement with that of CVs and DPVs. Hence, it is obviously that the significant enhancement of ERGO films associated with CVs and DPVs responses toward the probe could be attributed to the highly enlarged electroactive surface area and faster charge transfer in contrast to the corresponding bare electrode surface.

Fig. 4 Electrochemical impedance spectroscopy measurements of 5 mM K₃[Fe(CN)₆]/K₄[Fe(CN)₆] in 0.1 M KCl at the CPE (a), GE (b), GCE (c), respectively (black curves for the bare electrode, red curves for the ERGO modified electrode).

To further investigate the structure and electrocatalysis property of ERGO films modified carbon-based substrate electrodes, the electrochemical behaviours of 5.0 mM NaNO₂ containing 0.2 M PBS (pH = 7) on those ERGO films were investigated using CVs and DPVs. Fig. 5, black curves, depict typical CVs and DPVs of nitrite obtained at the bare CPE (a and d), GE (b and e) and GCE (c and f), respectively. Clearly, the bare CPE, GE, and GCE electrode shows typical anodic waves of the target of nitrite within the potential window, respectively. After the ERGO films modified, red curve shown in Fig. 5, the modified electrodes show a significantly electrocatalytical response of well-defined anodic peak toward the oxidation of nitrite with a large current, respectively (related parameters see Table S4†), indicating a good catalytic activity of ERGO films modified surface and thus facilitate the electrooxidation of nitrite, which can be attributed to its unique advantages such as more catalytic sites and the large electroactive surface area. As shown in Fig. S8,† from the trend-curves of potentials and currents of nitrite oxidation peak on different electrodes attained by CVs and DPVs, we can draw that the ERGO/CPE shows much higher anodic peak current than others, demonstrating an increased sensitivity toward nitrite oxidation. Besides, compared to the bare electrode, the oxidation potential of nitrite on three of ERGO films modified electrodes also shifted to more negative values, also indicating good electrocatalytic properties toward nitrite. The peak potential and currents with a considerable enhancement demonstrate that ERGO films can effectively accelerate the ET toward the target and the modified surface. It can be attributed to the enhanced surface and electrical conductivity of as-prepared modified electrodes, resulting adsorbing more probe molecules, thereby improving its electrochemical response.

Fig. 5 Cyclic voltammograms at the CPE (a), GE (b), GCE (c) and differential pulse voltammetrys at the CPE (d), GE (e), GCE (f) of 5 mM NaNO₂ containing in 0.2 M PBS (pH = 7), respectively (black and red curves for the bare and ERGO modified electrodes, respectively).

According to the above discussions, we can easily draw a conclusion that among three of ERGO films modified carbon-based electrodes, the ERGO/CPE possesses highly sensitivity toward nitrite electrochemical oxidation, strongly demonstrating that the stability and selective of ERGO films on CPE surface might be higher than the others. The fact is that not so? Because the electroactive surface area of each modified electrode is not the same. Therefore, to reasonably evaluate the electrocatalytic capability of three of ERGO films modified carbon-based electrodes toward nitrite electrochemical oxidation, the current density (J) vs. potential (E) curves of nitrite on different electrodes were proposed and presented in Fig. 6. As shown in Fig. 6a and b, whether CVs and DPVs J–E curves, the current density of the target with the trend of ERGO/CPE (297, 301 μA cm⁻²) < ERGO/GE (525, 525 μA cm⁻²) < ERGO/GCE (639, 550 μA cm⁻²), indicating that ERGO/GCE possesses higher current density than the others. To validate the reliability of this opinion, the stability of the as-prepared ERGO films was performed using amperometric I–t curve, shown in Fig. 6c. As can be seen, the constant amperometric response of 5.0 mM nitrite at the ERGO/GCE surface was achieved for 600 seconds. However, at the same condition, which were only 200 and 30 seconds for the ERGO/GE and ERGO/CPE, respectively, indicating the antifouling effects and stability of the ERGO/GCE towards nitrite oxidation.

Fig. 6 Cyclic voltammograms (a), differential pulse voltammetrys (b) and amperometric response (c) of 5 mM NaNO₂ containing in 0.2 M PBS (pH = 7) and at the different ERGO films modified electrodes (current density vs. potential).

Chronoamperometry technique is an effective tool for measuring the kinetics of target molecules via calculating the apparent rate constant (κ) and so on. Therefore, the chronoamperometry performances of three of ERGO films modified carbon-based electrodes were gained and compared with its corresponding bare electrode. Using double potential step method, chronoamperometric experiments were carried out by holding the electrode potential at 0.0 V for 10 s and forcing it to the nitrite oxidation potential, i.e., 0.88 V (for bare and ERGO modified CPE, GE, and GCE), until a steady state response was obtained. As observed in the current vs. time (I–t) curves of Fig. 7, it is clearly that the nitrite oxidation reaction is a diffusion controlled electron transfer process. According to the formula as follows (formula (3)) described in the literature,⁵¹ the apparent rate constant (κ) for the nitrite oxidation reaction can be determined via plotting the values of I_c/I_l vs. t^1/2.

where I_c/I_l is the ratio between the faradaic current measured after and before nitrite addition. C₀ is the concentration of nitrite in the bulk in M, k is the catalytic rate constant in M⁻¹ s⁻¹, and t is the time elapsed. For such a purpose, the apparent rate constants (k) calculated was 308, 107, and 42 M⁻¹ s⁻¹ for the bare CPE, GE, and GCE, respectively. After the ERGO films modified, the value of calculated k of the ERGO/CPE, ERGO/GE, and ERGO/GCE remarkably increased and was 430, 775, and 1019 M⁻¹ s⁻¹, respectively. Obviously, the apparent rate constants k of the nitrite oxidation reaction on the ERGO/GCE was significantly enlarged and was about 24 times than that on the bare electrode. However, it is only 7.2 and 1.4 times for the ERGO/GE and ERGO/CPE, respectively. The results demonstrate that ERGO films as-prepared under the same condition show significant and different electrocatalytic capability toward nitrite electrochemical oxidation. Among them, the ERGO/GCE possesses high electrocatalytic activity, stability and sensitivity, which might be attributed to the unique structural features.

Fig. 7 Amperometric response of with (black curves) and without (red curves) 5 mM NaNO₂ containing in 0.2 M PBS (pH = 7) at the bare CPE (a), bare GE (b), bare GCE (c), ERGO/CPE (d), ERGO/GE (e), ERGO/GCE (f), respectively. Insert shows I_c/I_l vs. t^1/2 plot obtained for the corresponding electrode.

Conclusions

In summary, a comparative study on the preparation of different ERGO films modified carbon-based substrate electrodes by one-step electrodeposition strategy using cyclic voltammograms were explored. The as-prepared ERGO/CPE, ERGO/GE, and ERGO/GCE were successfully characterized by SEM, XPS, Raman, and electrochemical methods. The results indicate that ERGO films can be indeed produced and modified on CPE, GE, and GCE surface from GO dispersions via a simple, fast and green electrochemical method proposed in this work. In addition, a significant difference of electrochemical stability and sensitivity of ERGO films on different substrate electrodes toward the probe of potassium prussiate and the target of nitrite were point out, which would be useful for the design of graphene-based electrochemical sensors and for the progress in this research area to continue.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (no. 21265009, 21265018), Program for Chang Jiang Scholars and Innovative Research Team, Ministry of Education, China (Grant no. IRT1283), Research Fund for the Doctoral Program of Higher Education of China (20126203120003).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Related characteristic (Raman, XPS) and electrochemical experiments, formula of cyclic voltammograms employed to calculate the electroactive surface area of the bare and ERGO films modified electrodes. See DOI: 10.1039/c5ra02737c

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